Many transistors formed over a glass substrate or the like are manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors including amorphous silicon have low field effect mobility, they can be formed over a larger glass substrate. On the other hand, although transistors including polycrystalline silicon have high field effect mobility, they are not suitable for being formed over a larger glass substrate.
In addition to a transistor formed using silicon, a technique in which a transistor is manufactured using an oxide semiconductor and applied to an electronic device or an optical device has attracted attention. For example, a technique of manufacturing a transistor by using zinc oxide or In—Ga—Zn—O-based oxide as oxide semiconductor, and of using the transistor for a switching element of a pixel of a display device and the like is disclosed in Patent Document 1 and Patent Document 2.
In an oxide semiconductor, part of hydrogen serves as a donor to release electrons as carriers. When the carrier concentration in the oxide semiconductor becomes high, a channel is formed in the transistor without voltage application to a gate. That is, the threshold voltage of a transistor shifts in the negative direction. It is difficult to remove hydrogen in the oxide semiconductor completely; therefore, it is also difficult to control the threshold voltage.
In Patent Document 3, it is disclosed that when hydrogen is added in an oxide semiconductor film, electrical conductivity of the oxide semiconductor is increased by four to five orders of magnitude approximately. Moreover, it is disclosed that hydrogen is diffused into the oxide semiconductor film from an insulating film which is in contact with the oxide semiconductor film.